The so-called soft handover means that when a mobile station wants to communicate with a new base station, at first it does not interrupt its connection with the original base station. During the handover process, the mobile station remains in communication with the original cell and a new cell at the same time in order to guarantee voice channels communicate fluently. The advantages of soft handover are:
(1) seamless handover;
(2) maintenance of conversation continuity;
(3) decreasing the possibility of lost conversations; and
(4) decreasing the transmit power of the mobile station at the handover area.
CDMA soft handover improves conversation quality and increases in some meaning system capacity. This is one of the main advantages of CDMA systems.
In present CDMA systems, soft handover is triggered by pilot signal strength variation measured by the mobile station. This means that the mobile station measures the pilot signal strength of a nearby base station, and uses the measured value as a basis to control a pilot neighbor list, a candidate list, and an active list and to activate soft handover. The pilot active list is a pilot set, which corresponds to base stations that are being connected to the mobile station. The pilot candidate list is a set of pilots, which are not in the pilot active list currently, but have sufficient strength, indicating that the service channel of the corresponding base stations can be successfully demodulated by the mobile station. The pilot neighbor list is a set of pilots that are neither in the pilot active list nor in the pilot candidate list currently, but under certain condition they will enter the pilot candidate list.
During handover, the mobile station establishes a service link with a new base station using a multiple single path receiving shunt of a Rake receiver. At the same time, the mobile station does not interrupt the service link of the original service base station, until the mobile station receives a signal from the original base station that it is less than a threshold value. Then the mobile station cuts its link with the original base station. FIG. 1 details this process:
(1) A user uses base station A for conversation. After the base station measures a pilot signal strength greater than a specific value T-ADD, the mobile station will send a pilot strength measure message to base station A and will turn pilot B to the pilot candidate list.
(2) Base station A receives the pilot strength measure message and sends a handover indication message using base station A and B to the mobile station.
(3) The mobile station moves the candidate pilot signal from the pilot candidate list to the pilot active list, begins to use the pilot active list (A, B), and sends a handover completed message.
(4) When the pilot A signal strength drops below a specific value T-DROP, the mobile station launches an active handover clearance timer of the pilot.
(5) After completing the time of the active handover clearance timer, the mobile station sends a pilot strength measure message.
(6) The base station receives the pilot strength measure message and sends back a corresponding handover indication message using only base station B to mobile station.
(7) The mobile station moves pilot signal A from the pilot active list to the pilot neighbor list and sends out a handover completed message.
During the soft handover process mentioned above, when a mobile station enters an overlap area of multiple base stations, because of multiple access interference and decreased signal strength, communication quality of the mobile station can decrease. At this time the target pilot signal strength of the nearby base station is near T-ADD but does not reach T-ADD, i.e., conditions of activating soft handover according to pilot signal strength are not satisfied. Therefore, it is possible that conversation quality worsens or is even lost before the mobile station enters soft handover. This eliminates one of the technical advantages of CDMA systems, namely, soft handover.